A feature which usually is aimed at when developing duplex stainless steels is the combination of good corrosion resistance and strength in quenched and annealed condition. For example, this feature was aimed at in the development of the ferritic-austenitic stainless steel which is disclosed in WO 88/02032. In applications, such as propeller and pump shafts, fans and highly loaded components, however, even higher mechanical strength in combination with excellent corrosion properties, particularly a high pitting corrosion resistance, is desirable to improve process efficiency, service life or weight of constructions.
There exist several ways to improve mechanical strength of stainless steels: cold deformation, nitrogen alloying or precipitation hardening (PH). The cold deformation technique has several limitations; particularly that it is useful only for products in rather small cross sections and which shall not be further deformed by bending or the like subsequent to the cold working operation. As far as the method of improving mechanical strength by nitrogen alloying is concerned, it does not give the desired increase in strength for duplex steels as nitrogen favours the softer phase in the structure, the austenite.